Polymer processing methods

Figure 1 – Blow molding process

Plastics are compounds mainly composed of polymers, which are a class of materials formed in general by long chains.  Due to interesting characteristics like durability, light weight, low cost and ease of processing, polymers are being increasingly used in several industries, from small devices for electronics to big parts used in the aerospace industry. Traditional processing techniques include extrusion (to make profiles), injection molding (a versatile technique to produce parts), blow-molding (to make hollow parts like plastic bottles), etc., as shown in figures 1 and 2. More recently, different types of processing and characterization techniques began to be developed or used to produce and analyse micro and nanostructures made of polymers. Electrospinning is one these promising technologies.

Figure 2 – Plastic objects produced by traditional processing techniques

Electrospinning: A technique to produce polymer fibers

Electrospinning is a technique through which we can obtain polymer fibers using an electric field as the driving force.

In the most common setup , a polymer is dissolved in a solvent and the resulting solution is placed in a syringe (figure 3). To maintain a flow, a syringe pump is normally used. Then the solution is charged by simply attaching a high voltage supply to the syringe’s needle. A grounded collector that can have different shapes is the last element to create the electric field. The charged droplet at the tip of the syringe needle will start to be deformed into a conical shape, known as Taylor cone. In optimal conditions of flow rate, voltage, distance to collector, solution viscosity and conductivity, a jet will erupt from the Taylor cone. As it travels to the grounded collector, the jet is rapidly stretched and charge interactions cause bending instabilities that help to make the jet even thinner. Solvent evaporation occurs very quickly and solid polymer fibers are formed right before touching the collector.

Figure 3 – Needle pump system for the electrospinning technique

The result is a porous structure, or a non-woven mat, with high superficial area that has potential for many applications (figure 4).

Figure 4 – Basic apparatus of electrospinning and typical resulting electrospun mat seen by Scanning Electron Microscopy (SEM)

Tailoring properties – blending

When there is a need for different properties, synthesizing new polymers can be expensive and time consuming. Blending (mixing) two or more of existing polymers is an interesting alternative to get a different range of properties according to the ratio polymer A/polymer B, in the simplest case. Blends can be either miscible, i.e., there is no phase separation between the original polymers or they can be immiscible and present phase separation between the polymers. The latter is much more common so that the dispersion of one polymer in the other will be important on determining the final properties. Given the aforementioned comparison between traditional processing techniques and microfabrication like electrospinning, the difference in order of magnitude brings new technical challenges. For example, if we take an immiscible blend of two polymers manufactured by injection, the size of the dispersed phase is usually in the range of a few micrometers. However, in electrospinning, the fiber diameter may be less than one micron and the dispersed phase can be nanometric. This effect, combined with greater superficial area, can amplify the synergetic effect.

Wetting properties of electrospun mats

If we consider that a large amount of polymer chains are on the surface of electrospun mats, then the control of surface properties becomes critical. For instance, these mats have a naturally rough surface that can amplify the natural hydrophobic (“water fearing” – Figure 5 a) or hydrophilic (“water loving” – Figure 6 a) characters of a polymer surface. These properties can be quantified in terms of the contact angle made by a drop of water on the surface (Figure 5 b). If the angle is higher than ninety degrees, the surface is considered hydrophobic. If it is smaller than ninety degrees, the surface is hydrophilic. In our lab, we were able to produce a highly hydrophobic electrospun mat with contact angle of about 140° (Figures 5 a-b). In some cases from the literature, superhydrophobicity (contact angle higher than 150°) may be achieved, and a drop of water may even bounce on the surface.

After producing the hydrophobic mat, a second polymer was introduced to the solution prior to electrospinning, in order to produce a hydrophilic surface. The result (Figures 6 a-b) was a superhydrophilic mat (contact angle equal to zero degrees) with fast water absorption. If we analyse the mat and fiber morphology presented in Figures 5 c-d and Figures 6 c-d, it is clear that the addition of a second polymer reduced the fiber diameter and changed the surface of individual fibers, which was smooth for the hydrophobic mat and rough in the hydrophilic case. Now the focus is to deepen the understanding of this phenomenon to tailor the wetting properties of electrospun mats by choosing optimal materials and processing parameters.

Figure 5 – Hydrophobic fibers: (a) general view of water droplets on the electrospun surfaces; (b) contact angle measurement; (c) Electrospun mat surface observed by SEM (scale bar: 200 µm) and (d) individual fiber surface (scale bar: 2 µm)

Figure 6 – Superhydrophilic fibers: (a) general view of water droplets on the electrospun surfaces; (b) contact angle measurement; (c) Electrospun mat surface observed by SEM (scale bar: 200 µm) and (d) individual fiber surface (scale bar: 2 µm)

By varying the parameters during the electrospinning process, it is possible to obtain droplets or fibers, beads on string morphology, fibers with different diameters, etc. This will affect the final water contact angle. The type of polymer or polymers used is also a key factor that will influence the shape and properties of the fibers.

Membranes

Wetting properties are important for several possible industries. In filtration or separation membranes, for example, controlling the fiber surface properties may allow the separation of two different liquids like oil and water. Superhydrophobic mats can also produce self-cleaning surfaces, while hydrophilic mats can have nonfouling surfaces (low protein adsorption) interesting for biomedical applications.

Conclusion

Regardless of the possible application, the main focus of our research laboratory is to understand how the processing parameters and materials properties will determine the material’s microstructure and consequently the final properties (mechanical, electrical, wetting). When we control the microstructure, we can manage the properties and then define the application possibilities.

ETS Research Chair on Blends and Nanocomposites Based on Thermoplastics

This video presents the new ÉTS Research Chair on Blends and Nanocomposites Based on Thermoplastics directed by professor Nicole Demarquette (video in french only).